1. Field of the Invention
The present invention generally relates to an electromagnetic valve having a valve element which is arranged to close a fluid passage upon electrical energization of the electromagnetic valve. More particularly, the present invention is concerned with an improvement of the electromagnetic valve of such a structure in which the fluid under control (i.e., controlled fluid) is incapable of flowing around a coil upon electrical energization thereof.
2. Related Art
A three-way electromagnetic valve which has an input port, an output port and a drain or discharge port and is designed to changeover the fluid passages formed between the ports in response to electrical energization and deenergization of a coil is heretofore known. The electromagnetic valve includes in general a plunger housed within a plunger chamber and driven by the coil, a rod connected to the plunger at an end thereof, a first valve element of a conical shape formed in the rod at an intermediate portion thereof and a second valve element disposed so as to contact with a tip end of the rod.
In the state where the coil is not electrically deenergized, the second ball-like shaped valve element is pressed against a second valve seat to close the passage between the input port and the output port whereas the first valve element is detached from the first valve seat to place the output port and the discharge port in communication with each other.
On the other hand, when the coil is electrically energized, the plunger is driven or actuated to force the first valve element to bear on the first valve seat to thereby block the passage extending between the output port and the drain or discharge port while the second valve element is pushed downwards to move the second valve element from the second valve seat, whereby the input port and the output port are hydraulically communicated to each other. For more particulars, reference may have to be made to Japanese Patent No. 3219611.
As is apparent from the above, in the conventional electromagnetic valve of the structure mentioned above, the passage extending between the output port and the drain or discharge port is closed when the coil is electrically energized. In this state, no fluid under control can flow around or in the vicinity of the coil. Consequently, when the state in which the coil is electrically energized with the passage between the output port and the discharge port being blocked continuous for an extend time, there will arise a problem that the temperature of the coil increases. When the temperature of the coil rises, the electric resistance of the coil increases, causing the electric current flowing through the coil to be decreased, as a result of which the magnetic attraction for driving the plunger is lowered, giving rise to a problem. In order to increase the plunger attracting force even when the coil temperature rises, the coil has to be implemented in a large size or the conductor material of the coil has to be correspondingly selected or changed.